He could be witnessing something that would stand many years to come or something that would fall down tomorrow.

In certain domains, you can make the inference that you are observing a thing at a random point in its lifetime, and this is what Gott did.

He figured that if he’s witnessing it 50% of the way through its lifetime, then it would stand another 8 years (Because it has been up 8 years and is halfway through its life).

He figured that if he’s witnessing it 5% of the way through its lifetime, then it would stand another 152 years. (Because if 8 years is 5% of its life, then 160 years is 100% of its life. Since it has been up for 8 years, it has 152 years left).

He figured that if he’s witnessing it 95% of the way through its lifetime, then it would stand another 5 months. (Because if 8 years is 5% of its life then 8.42 years is 100% of its life, Since it has already been up 8 years, it has .42 years, or five months, left).

In this way, Gott came up with a principle for estimating confidence intervals for the lifetimes of certain classes of things. A 95% CI for the lifetime of the Berlin Wall would be 8.42 to 160 years, which contains the age of the wall (about 28 years) when it came down.